What is a sensible scale to work with?
By default one BU = one meter.  This is a pretty reasonable size to go by as a rough guideline.

If you are modeling artistically you probably don't need to worry about units too much, but you should try to keep everything from ending up very small or very large, which could cause clipping issues in the viewport, making it difficult to work with. Though if this does happen, it's not hard to scale everything up/down.

If you are working precisely with units, then you can always set them the way you want in Properties > Scene > Units: